Screening device

ABSTRACT

A screening device for pulp production includes a cylindrical container for continuous flow-through of a fiber suspension and further includes an inlet to the container and an outlet for the fiber suspension, a moveable support member mounted in the container and which supports hollow screening members for vertical movement; a tubular enclosure open at opposite ends is mounted on the supporting member which enclosure has a moveable piston mounted therein, a hydraulic cylinder being connected to the piston to move the piston in a portion of the enclosure with the hydraulic drive mounted outside of said container with transmission members connected to said supporting member which will therefore absorb the reaction forces generated in moving the piston.

FIELD OF THE INVENTION

The present invention relates to a screening device in a standing,preferably cylindrical, container for continuous flow-through of a fibresuspension, wherein the liquid content in the said fibre suspension isaltered with the aid of the screening device.

BACKGROUND OF THE INVENTION

Screens of the abovementioned category have been known for a long time.The invention will be illustrated below by referring to a specific fieldof application for such screening devices, namely the pulp industry andin this regard particularly in connection with the washing of pulp.

Through U.S. Pat. No. 5,027,620 (Kamyr) a screening device of this typeis already known which comprises a cylindrical housing with: an inletand an outlet for pulp, a movable supporting member which supportshollow screening members, means for vertical movement of the saidsupporting member, a container which is arranged on the said supportingmember, which container is open at both ends and has a movable pistonmember arranged inside it together with means for moving the said pistonmember. In this known screening device, the drive unit for the verticalmovement of said piston member, in the form of an hydraulic piston unit,is arranged inside the housing of the screening device, for moving thepiston member. However, this arrangement has been found to entailcertain disadvantages, since it has been found difficult completely toavoid leakage of hydraulic fluid, which has a negative effect on thequality of the pulp. Consequently a goal is to attempt to eliminatecompletely the risk of such leakages.

The solution disclosed in U.S. Pat. No. 4,971,694 (Kamyr) solves thelatter problem by placing the hydraulic unit entirely outside thehousing of the screening device. However, problems of another type havebeen found to arise with a device of the latter type. These problems aremore related to the design of the apparatus and are due to the fact thatthe drive unit, which moves the piston member, exerts very large forces.This is because, in such a screening device, relatively large liquidvolumes (about 1 m³) have to be moved within relatively short times(about 2 sec). Consequently, rigid structures are needed for mountingthe hydraulic unit. In a device according to U.S. Pat. No. 4,971,694,this structure consists of a stout beam which, simply for this reason,has been arranged straight through the housing at its upper end. Despiteits relatively generous dimensions, the beam bends markedly when thehydraulic unit is operating, which can be found to be troublesome. Forvarious reasons, therefore, this latter solution cannot be considered tobe ideal.

SUMMARY OF THE INVENTION

An object of the present invention is to solve the problems that wereinitially mentioned above in such a way that the disadvantages arisingin association with a solution according to U.S. Pat. No. 4,971,694 areentirely avoided.

This object is achieved with the aid of a screening device, wherein thesaid means for moving the piston member comprises a drive unit arrangedoutside the housing, preferably an hydraulic piston unit, from which thereaction forces counteracting the piston member via transmission membersact on the supporting member for the screening members.

By using the supporting member for the screening members to absorb thereaction forces from the hydraulic piston unit, the beam according toU.S. Pat. No. 4,971,694 can be eliminated in an economical manner byutilising this rigid structure, i.e. the supporting member for thescreening members, present in the screening device.

A further advantage of a solution according to the invention is that thepoint of application of the reaction forces can be placed in a planewhich is in common with the points of application from the hydraulicpiston units which drive the supporting member in its reciprocalvertical movement. This eliminates the risk that a piston rod and othertransmission members for the centrally located components might besubject to flexural stresses if the supporting member were to tilt toany extent.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be explained in more detail below with reference tothe attached figures, in which:

FIG. 1 shows the essential details of a screening device according tothe invention in vertical longitudinal section, and

FIG. 2 shows a specific solution according to the invention forarranging the point of application of the reaction forces from thehydraulic piston unit.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a screening device for washing pulp. The screening deviceconsists of a housing 1 inside which the pulp M is fed from below andupwards through its lower cavity 2. After washing, the pulp is fed outthrough an outlet 3. Screening members 4 in the form of severalring-shaped, concentric, hollow elements 4A, 4B, are arranged on thesupporting member 5. The supporting member 5 consists of several hollowarms, inside which are formed channels 5A, 5B, and is movable in thevertical direction with the aid of hydraulic pistons 6 which are fixedto the lower part of the housing and which, via a piston rod 6A, act ona ball joint 6B at the outer periphery of the supporting member 5. Abovethe screening assembly 4 there are rotating arms 7 fixed to a rotarysleeve 8. The arms 7 support scrapers 7A and members 7B for supplyingwashing liquid. The rotary sleeve 8 is mounted inside a cylindrical unit17 and is rotated by means of motors 10 and a transmission unit 9. Thesleeve 8 is held centred with the aid of a bearing unit 11 supported bya console.

Underneath the supporting member 5 there is a cylindrical container 12,which is open both at the bottom and at the top. A piston member 13 isarranged movably inside the container 12. The piston member 13 is movedwith the aid of an hydraulic unit 15 by means of an extended piston rod14. The cylinder for the hydraulic unit 15 is arranged above acylindrical housing 16 which is telescopically displaceable inside alower, larger cylindrical housing unit 17 which is fixed at the upperend of the screening device. The upper cylindrical unit 16 is firmlyconnected 20 to an extended pipe 19 which is connected to the supportingmember 5 via a specially constructed ball joint 25.

As already indicated, liquid can be supplied with the aid of the members7B arranged on the rotating arm 7. Removal of liquid can take place fromthe cavity 5A inside the supporting member 5 through several outlets 22and in between via telescopically displaceable cylinder housings 23, 24.

FIG. 2 shows in detail the special design of the mounting point 25 forthe pipe 19 which transmits the reaction forces from the hydraulicpiston unit 15 to the supporting member 5. At the very bottom of thepipe 19, a rotationally symmetrical sphere 19A is arranged inside acorresponding rotationally symmetrical female part 25D lying outside it.This female part 25D is connected, with the aid of suitable means 25E,for example screws, to a central part 5C of the supporting member 5. Therod 14 for the piston member 13 is arranged centrally inside the saidjoint 25 with the aid of a ball joint packing 25A, 25B inside which therod 14 can slide in a sealing manner and can even be permitted to flex.

The screening device according to the invention functions in a knownmanner by the pulp which is to be washed being fed in at the bottom 2and moving continuously upwards through the housing 1. Washing liquid issupplied with the aid of the spraying members 7B, and liquid is drawnoff by drawing off liquid through the screening rings 4 down into thecavity 5A inside the supporting members 5 and out through the orifice22. Pulp M is the whole time fed out continuously through the outlet 3with the aid of the rotating scraping members 7A. During the greaterpart of the described process, the supporting member 5 accompanies thepulp in an upwardly directed movement by the hydraulic units 6continuously moving the supporting member 5 upwards. Once the screeningmembers 4 and supporting members 5 have reached their upper terminalposition, they must be quickly moved down to their lower terminalposition. It is important that the pulp bed which has built up round thescreening rings is not dragged down (destroyed) in connection with thisdownwardly directed movement.

By flushing liquid backwards, i.e. upwards and out, through thescreening rings 4A a short while before the screening assembly 4 andsupporting members 5 are moved downwards, a liquid layer can be built upbetween the pulp bed and the screening rings. This in turn makes itpossible to carry out the said rapid movement without the pulp bed beingcarried along. This back-flushing of liquid takes place using the liquidwhich is collected inside the container 12 underneath the supportingmember 5. By means of the hydraulic unit 15, a rapid movement of liquidcan therefore be effected out of the container 12, through the centralchannel 5B and thence out into the cavities 5A inside the supportingarms 5. From here the liquid finally flows into the screening members 4Afrom the bottom and continues up through these, whereby the saiddesirable liquid layer is built up between the pulp bed and thescreening member 4A. The reaction forces which arise in connection withthe back-flushing will therefore, with, a solution according to theinvention, directly act against the supporting member 5 in its centralpart 5C.

The invention is not limited by that which has been demonstrated above,but can be varied within the scope of the subsequent patent claims.Thus, it is obvious to the person skilled in the art that the describedsolution can also be applied to screening devices intended for otherthings than pulp and which operate according to similar principles.Additionally, it is obvious that the cylindrical shape is a preferredembodiment, but that other shapes can also be used, such as, forexample, housings which are rectangular at least in part, as can unitsadapted to them.

For the rest, it should be mentioned that the invention is also suitablefor use in so-called double diffusers (see, for example, EP 377,427).However, in order to be able to keep the peripheral application points6B in the same plane as the piston mounting 25 in connection with adiffuser of this type as well, the smaller of the two container unitsfor the back-flushing must be moved down so that it is located insidethe larger unit. This then provides space for the said mounting.

I claim:
 1. A screening device includes a standing cylindrical containerhaving an inlet and outlet for continuous flow-through of a fibresuspension, said device including a moveable supporting member disposedin said container and which supports hollow screening members forvertical movement with said supporting member, an enclosure connected tosaid supporting member having opposite open ends and a moveable pistonmounted in said enclosure, means for moving said piston comprising ahydraulic drive unit mounted outside of said container and transmissionmeans connected between said hydraulic unit and said supporting memberfor counteracting the reaction force of the hydraulic unit onto saidsupporting member.
 2. The device as claimed in claim 1 wherein saiddevice includes hydraulic units for driving said supporting membervertically and which each act on said supporting member at a selectedapplication point, said transmission means includes transmission membershaving a point of application on said supporting member whichsubstantially lie in the same plane as the application points of saidhydraulic units which drive said supporting member vertically.
 3. Thedevice as claimed in claim 1 wherein said screening member comprises aplurality of circular concentric ring elements.
 4. The device as claimedin claim 3 wherein a rotary member is mounted in said container andsupports means for supplying washing liquid between said ring elements.